The present invention relates to an irradiation system, and more particularly to a system and method for irradiating product in a manner that improves the uniformity of the irradiation dose delivered to the product.
Irradiation technology for medical and food sterilization has been scientifically understood for many years dating back to the 1940""s. The increasing concern for food safety as well as safe, effective medical sterilization has resulted in growing interest and recently expanded government regulatory approval of irradiation technology for these applications. United States Government regulatory agencies have recently approved the use of irradiation processing of red meat in general and ground meat in particular. Ground meat such as ground beef is of particular concern for risk of food borne illness due to the fact that contaminants introduced during processing may be mixed throughout the product including the extreme product interior which receives the least amount of heat during cooking. Irradiation provides a very effective means of reducing the population of such harmful pathogens.
Various types of radiation sources are approved for the treatment of food products including gamma sources such as radioactive cobalt 60, accelerated electrons with energy up to 10 MeV, and x-rays from electron accelerators of up to 5 MeV. Electron beam and x-ray machine generated sources are becoming increasingly popular due to their flexibility and a general consumer preference to avoid radioactive materials.
The beneficial effects of irradiation of food are caused by the absorption of ionizing energy that results in the breaking of a small percentage of the molecular bonds of molecules in the product. Most of the molecules in food are relatively small and are therefore unaffected. The DNA in bacteria, however, is a very large molecule and is highly likely to be broken and rendered unable to replicate.
FIG. 1 is a graph of exemplary percentage depth-dose curves showing the reduction of radiation intensity due to absorption of radiation in water (which is a relatively accurate model for radiation absorption in food products). Curve 10 is a percentage depth-dose curve for 1.8 MeV electrons, curve 12 is a percentage depth-dose curve for 4.7 MeV electrons, and curve 14 is a percentage depth-dose curve for 10.6 MeV electrons. For all of the electron energies, the radiation intensity increases to a maximum at a distance somewhat interior to the surface of the product due to scatter emission of radiation from electron collisions with food molecules. After the maximum is achieved, absorption causes the relative intensity to begin to fall off until virtually all of the radiation has been absorbed. At the xe2x80x9ctailsxe2x80x9d of the depth-dose chart the intensity is much less than the maximum, but still results in an incremental amount of beneficial irradiation. Single sided application of radiation that is required to maintain a moderate ratio between maximum and minimum exposure must necessarily waste most of this tail of radiation intensity.
Curve 12 of FIG. 1 illustrates that the percentage depth-dose for 4.7 MeV electrons is approximately 50% of its maximum value at a penetration depth of about 2.0 centimeters or 0.8 inches. Exposure of food of this thickness would result in a maximum/minimum dose ratio of 1/0.5=2.0. The portion of the beam power that is not absorbed would pass through the material and be wasted. The preferred solution to this inefficient use of the ionizing radiation is to expose the product to the electron beam from two sides. FIG. 2 is a graph of an exemplary depth-dose curve for two sided 4.7 MeV exposure of product having a 4.0 centimeter or 1.57 inch thickness. The depth exposed is substantially greater than for single sided exposure, and the maximum/minimum ratio is substantially lower, resulting in more precise and consistent product exposure.
While two sided irradiation is preferred for maximum efficiency and most consistent exposure, generation of two sided radiation can be problematic. The typical solutions are to either pass product through the radiation source once per side, which requires twice as long to process and may not be viable for products that cannot be flipped over due to material redistribution, or to create two independent accelerators which is costly and complex.
Electron accelerators of several types are known in the art. A preferred electron accelerator for irradiation applications is the well known linear accelerator or LINAC, which employs a high power microwave source driving a specially constructed waveguide to accelerate electrons by electromagnetic induction. A preferred LINAC operation methodology is pulsed operation, whereby a relatively short, high intensity pulse of accelerated electrons is generated at a selected repetition rate. The timing and magnitude of this pulse of accelerated electrons may be controlled by a computer control system.
The stream of accelerated electrons emerging from a typical LINAC is concentrated into a narrow beam approximately 0.5 centimeters in diameter, which is much too small and intense to apply directly to material to be processed. Prior art systems typically shape and spread the beam by passing it through a quadrupole magnet which spreads the beam in both the vertical and horizontal dimensions in a manner analogous to an optical lens. FIG. 3 is a diagram illustrating a typical spread beam intensity distribution, which takes the shape of elliptical profile 20. The intensity profile corresponds generally to bell shaped distributions 24 and 26 centered about the vertical and horizontal axes of symmetry. Line 22 surrounding elliptical profile 20 corresponds to the points where the intensity is at halfpower (or xe2x88x923 db) from maximum. A two-dimensional bell shaped distribution corresponding to a normalized raised cosine function:
f(x,y)=(1+cos(x))*(1+cos(y))/4
is represented numerically by the table shown in FIG. 4.
Prior art irradiation systems, such as the system disclosed in published PCT Application No. WO01/26135 filed by Mitec Incorporated, the same assignee as the present application, apply a series of 50% overlapping pulses of accelerated electrons formed in an intensity profile according to the elliptical pattern shown in FIGS. 3 and 4. Various points in FIG. 4 are shown with a box around them, including the center point with normalized intensity of 1.00, the 25% points (halfway between the center point and the 0.50 intensity points) with a normalized intensity of 0.73, and a set of points forming a generally elliptical shape surrounding the center point. These points represent normalized intensity values between 0.47 and 0.53 (approximately xe2x88x923 db) and correspond generally to the elliptical shape shown in FIG. 3. A 50% overlap results in a constant intensity distribution along the axis of symmetry. With 50% overlap in both the vertical and horizontal dimensions, the resultant two dimensional exposure is four times the single pulse peak exposure. This distribution, however, is not exactly constant off the axes of symmetry. The greatest deviation is observed at the 25% points. With 50% overlapping vertical and horizontal exposure, the normalized exposure at these points is:
0.73xc3x974=3.44
which is 14% less than the nominal xe2x80x9con-axisxe2x80x9d exposure. When an important performance criterion for irradiation exposure is uniformity of dose, this exposure variation contributes directly to an increased maximum/minimum dose ratio, and is undesirable.
FIG. 5 is a schematic diagram illustrating a single accelerator, two sided irradiation system 30 having a structure similar to that disclosed in published PCT Application No. WO01/26135. Irradiation system 30 includes quadrupole magnet 32, upper deflection magnet 34 and lower deflection magnet 36 for direction of electrons toward material 38. The paths that accelerated electrons may be directed by relatively constant currents in deflection magnets 34 and 36 from a single accelerator to two sides of material to be processed are illustrated by dotted lines. A benefit of the system of FIG. 5 is that relatively few magnets are required to direct the accelerated electrons to the two opposite sides of material. There is, however, a substantial difference in the path lengths that electrons must travel from deflection magnets 34 and 36 to material 38 being processed. Since deflection electromagnets operate on accelerated electrons by displacing their path in an angle proportional to the magnetic field, the field required to deflect electrons to a selected position must be set to a predetermined value. This predetermined value may be controlled by a computer driving a relatively constant current into the magnet to direct the electrons to the correct location. Unfortunately, if the beam spot is formed by a typical quadrupole magnet such as quadrupole magnet 32, the formed elliptical beam spot consists of diverging rays of electron paths, so the elliptical spot will be larger in an amount proportional to the path length. In the illustration of FIG. 5, an exemplary physical size for the total height of the apparatus may be 72 inches or more, so the path length may vary from as little as 24 inches for the inner downward path to more than 100 inches for the outer upward path. This 4:1 length ratio would cause a corresponding 4:1 increase in the beam divergence and resulting elliptical spot size. The increased spot size may be so large that the width of the scan horn (not shown in FIG. 5) may have to be increased to provide an unrestricted path for the accelerated electrons to be directed to material 38 to be processed. The scan horn is typically constructed of very rigid stainless steel and provides a high vacuum environment for the propagation of electrons with minimum attenuation. It is desirable for the interior volume of the scan horn to be minimized to minimize the required vacuum pump capacity
It would be desirable to provide a system for applying radiation to two opposite sides of articles from a single radiation source with precise uniformity of the dose applied to the articles. The present invention is a cost effective method and apparatus utilizing a single pulsed accelerated electron source and simple electron beam manipulation elements to process, form and direct a stream of electrons to material to be processed with controlled, uniform dosage.
The present invention is a system and method for providing irradiation to material. An electron beam is shaped into a profile having a substantially rectangular intensity distribution. The profile is deflected onto the material in a pattern with substantial overlap in a first dimension and without substantial overlap in a second dimension. In an exemplary embodiment, irradiation is provided to the material from first and second opposite sides.